The present disclosure pertains to the field of signal transfer between components. More particularly, the present disclosure pertains to slew rate control of line termination devices used on bi-directional transmission lines.
Termination techniques are employed to improve the signal quality of communications between components by reducing signal degradation due to transmission line artifacts such as reflection or ringing. One termination technique is known as source termination.
Source termination is illustrated in FIG. 1a. In source termination, a transmission line, 113, is terminated through use of a resistance, 115, at the driver or source end, 110, of the transmission line. Typically this resistance is provided by the active device in the driver. When a signal is transmitted by a driver, 112, it propagates along the transmission line until it reaches a receiver, 114. A reflection occurs at the receiver end of the transmission line, 111, due to an impedance discontinuity and the reflected signal propagates back along the transmission line in a direction toward the driver. When the reflected signal reaches the driver, it is absorbed by the source terminator, 115.
One problem with source termination, is that it limits the frequency at which signals can be transmitted, since a driver must wait for previous signal reflections to propagate before transmitting new signals. If a new signal is transmitted before the reflections die down, signal degradation due to jitter may result.
An alternative technique is called center-tapped termination (CTT). The CTT technique is illustrated in FIGS. 1b and 1c. CTT is used to absorb or avoid reflections at the receiver end of the transmission line, 121, by providing two equal resistances: one, 126, tied to a voltage source VCC and the other, 127, tied to a ground voltage source. Thus the receiver end of the transmission line is biased to a central voltage of VCC/2.
Since a component, 130 or 131, on a bi-directional transmission line, 133, may act as a receiver at one time and as a driver at another time, the CTT device 135 may be turned off when the component is acting as a driver 132, and the CCT device 136 may be turned on when the component is acting as a receiver 134.
One problem with this technique is that during the time when receiving devices are turning on their termination CTT devices and a driver is turning off its CTT devices a glitch may occur. A graph of the voltage levels on a transmission line as seen by a receiver is shown in FIG. 2. A glitch, 202, results from a new CTT device turning on as the old driver turns off. When the driver turns off the line may tend to float to the bias voltage level of VCC/2. One danger is that receivers on the transmission line may mistake the glitch, 202, for a new valid signal level, 204, transmitted by the new driver.
Another technique used to improve signal quality is known as slew rate control. An example of one prior method, discussed in U.S. Pat. No. 5,977,790, is shown in FIG. 3. In the circuit shown, a signal, SEL, is used to select between two driver circuits. One of the driver circuits employs devices M1 and M2 to drive the voltage accumulated at PAD to the same logic level as the value received at the input, DATA. The other driver circuit employs devices M5 and M6 to drive the desired voltage accumulated at PAD, but through a resistance, RT, thereby producing a selected slew rate, which is different than that of the first driver circuit.